A change from invalid to valid is governed by the Ahmedabad principle, BIM34025.

Is the old basis of computing the profits a VALID basis? Guidance on valid and invalid basis is at BIM34020.

Yes

Has there been a change in basis or simply a refinement of the existing method, see BIM34050.

Yes, change in basis

Is the change a change in accounting policy? BIM34045.

No

Changes in tax computation adjustments see BIM34090.

No

No adjustment is necessary, BIM34015.

Yes

Changes in accounting policy see BIM34070.